1. Field Of The Invention
The invention is generally in the field of communication satellites and more specifically relates to on-board satellite switched multiple access systems.
2. Description Of The Prior Art
Present commercial communications satellites function as multiple access microwave repeaters for interconnecting communications signals between several earth stations. As presently constituted, communication satellites use a global coverage antenna common to multiple transponders, each comprised of a receiver and a transmitter. Generally, frequency modulated communications signals are transmitted between the earth stations and the satellite using frequency division multiple access, FDMA. FDMA grants the satellite the opportunity to simultaneously carry several channels of information. Channels are defined by frequencies, and the number of channels which can be carried is limited to the frequency spectrum allocated to the satellite system. At present, satellite systems are constrained to a 500 mHz band.
Space division multiplexing, SDMA, is a known technique for expanding the available number of communications channels. In an SDMA system narrow beam, directional antennas, known as spot beam antennas, are dedicated to specific geographical zones, with one or more earth stations being assigned to each zone. For each geographical zone, there is an uplink with its corresponding satellite on-board spot beam receive antenna and a downlink with its corresponding satellite on-board spot beam transmit antenna. A transponder is provided for each common zone uplink and downlink antenna pair. While the uplink and downlink for a geographical zone may be served by separate receive and transmit spot beam antennas, a single spot beam antenna can replace the two separate antennas. When a single spot beam antenna is used, the uplink and downlink signals are distinguished using conventional diversity techniques, such as frequency or polarization diversity.
In the simplest form of an SDMA system, the interconnections between uplinks and the downlinks are fixed. Considering four uplinks A, B, C and D and four downlinks W, X, Y and Z, a fixed interconnection system would have, for example, uplink A connected to downlink W, uplink B connected to downlink X, uplink C connected to downlink Y and uplink D connected to downlink Z.
Greater flexibility is achieved if the interconnections between uplinks and downlinks can be varied at will. Such an arrangement, known as a satellite switched multiple access system is described in U.S. Pat. No. 3,711,855, Schmidt, et al., issued Jan. 16, 1973, and U.S. Pat. No. 3,928,804, Schmidt, et al., issued Dec. 23, 1975. Such satellite systems operate in time division multiple access, TDMA, and are thus denoted SS-TDMA systems, meaning satellite switched, time division multiple access systems.
In an SS-TDMA system, the uplinks are selectively connected to each of the downlinks through a matrix of microwave switches. An example of such a microwave switch is described in U.S. Pat. No. 3,813,497, Wachs, issued May 28, 1974. Each uplink is connected to a row of switching elements, such as PIN diodes, with each diode in a row being connected via matrix columns to a downlink. The earth stations are operated in a time division mode sending their communications signals to the satellite in designated time slots in accordance with the signal destinations. Earth station transmissions are synchronized to the satellite on-board switching of the matrix crosspoint switching elements so that communications signals from the earth stations are directed to the proper downlinks.
Conventional SS-TDMA systems are presently configured for international telecommunications using point-to-point interconnections. Point-to-point means that at any moment, each uplink communications signal connects to a single downlink. That is, at any moment in time, only one crosspoint for each row may be conductive. This arrangement distinguishes from the broadcast concept of earth station interconnection through a satellite link. Under the broadcast concept, also referred to as point-to-multipoint communication, an earth station communicates with several other earth stations simultaneously. In conventional FM/FDMA systems, broadcast-type interconnections are easily effected since global coverage antennas are used. Each earth station may simultaneously receive and recognize all satellite relayed signals since these signals transmitted by global coverage antennas are frequency discriminated. A similar broadcast capability is inherent in TDMA systems using global coverage antennas. However, once the satellite is configured to operate in the SS-TDMA mode by providing the satellite with spot beam antennas, known switching systems preclude broadcast interconnection since only point-to-point interconnection is feasible.
In applicant's co-pending application Ser. No. 826,625, filed Aug. 22, 1977, an improved SS-TDMA system is described wherein point-to-multipoint interconnection capability is provided during, what is termed, static mode operation. As explained in said copending application, operational satellites function in FM/FDMA using global coverage antennas to communicate between the earth stations and the satellite. In such systems, signal discrimination is based solely on frequency diversity. TDMA satellites will be the next generation operational satellites, with such satellites operating in SS-TDMA. However, during the change-over period when both FDMA and TDMA will be used, satellites will be required to provide broadcast capability for FDMA signals as well as point-to-point dynamic operating capability for TDMA communication. The broadcast capability provided in copending application Ser. No. 826,625 is quite adequate for FDMA operation and indeed defines an improvement over prior systems. However, broadcast operation in response to TDMA signals is not available in this prior system.
It has since been determined that it would be advantageous to provide an SS-TDMA system with point-to-multipoint operating capability during dynamic TDMA operation when the switching matrix interconnection pattern is subject to rapid change, such as once per frame unit. It is to such an improved system that the present invention is directed.